LITHIUM FERRITE CARRIER

Abstract: A non-stoichiometric lithium ferrite powder having a compositional range represented by the formula: [(Li2O).25 (Fe2O3).25]x (Fe2O3)1.00-x where .35 </= x </= .50 mole percent, provides an environmentally safe carrier.

Description: LITHIUM FERRITE CARRIER BACKGROUND OF THE INVENTION The present invention relates to a magnetic carrier for use with electrophotographic development equipment and, more particularly, to an environmentally benign lithium ferrite carrier having a non-stoichiometric composition. Carriers in the form of powder are used to transfer toner particles in electrophotographic development equipment, for example, in photocopying machines and most recently in laser printers. Typically, such carriers are ferrites or ferrite powders in combination with various metals, for example, nickel, zinc, or copper. Numerous patents have issued directed to various ferrite carrier compositions including the following: limura et al., U.S. Patent No. 4,623,603; Honjo et al., U.S. Patent No. 4,598,034; Tachibana et al., U.S. Patent No. 4,898,801 , Imamura et al., U.S. Patent No. 4,485,162; and Jones, U.S. Patent No. 3,929,657. The prior art patents teach both single component and dual component ferrite carriers. These patents also teach various crystalline structures for the carriers. In general, these patents teach the utilization of stoichiometric compositions of the various metals with ferrites. Additionally, these patents teach various processes for the manufacture of such carriers. The research with respect to such carriers has been an ongoing effort and most recently it has been recognized that many ferrite carrier powders are produced with compositions that contain elements that may be regarded as hazardous to the environment, such as the metals: nickel, copper and zinc. Thus, there has developed a need to provide an environmentally benign carrier which may be safely and easily disposed once it has served a useful life. The present invention is directed to an environmentally safe carrier which is also an efficient and effective substitute for prior art carriers not considered to be as environmentally safe. SUMMARY OF THE INVENTION In a principal aspect, the present invention comprises a carrier for electro¬ photographic developing comprising a generally non-stoichiometric lithium ferrite powder having a particular compositional range. The carrier has a substantially spinel crystalline structure and may be formed in a generally spherical shaped magnetic core configuration for use in pre-existing conventional electrophotographic equipment. Thus it is an object of the invention to provide an improved electrophotographic development carrier material which is environmentally safe or benign. It is a further object of the invention to provide an electrophotographic carrier which is as useful as prior art carriers that incorporate other metal elements. Yet another object of the invention is to provide an electrophotographic carrier which is a non-stoichiometric lithium ferrite compound. A further object of the invention is to provide a lithium ferrite powder for use as a carrier having a form and being in a condition for use with electrophotographic equipment already in service. Another object of the invention is to provide an electrophotographic development carrier comprised of lithium ferrites having a range of composition. Yet a further object of the invention is to provide a method for manufacture of a lithium ferrite carrier having a spinel crystalline structure and which is useful in electro- photographic processes. These and other objects, advantages and features of the invention will be set forth in the detailed description which follows. BRIEF DESCRIPTION OF THE DRAWING In the detailed description which follows, reference will be made to the drawing comprised of the following figures: FIGURE 1 is a phase diagram for lithium ferrite compositions illustrating the range of the composition of the carrier of the present invention; FIGURE 2 is a photomicrograph of the carrier of Example No. 1 of the invention at 50 magnification; FIGURE 3 is a photomicrograph of the carrier of Example No. 1 of the invention at 200 magnification; FIGURE 4 is a photomicrograph of the carrier of Example No. 2 of the invention at 50 magnification; FIGURE 5 is a photomicrograph of the carrier of Example No. 2 of the invention at 200 magnification; FIGURE 6 is a photomicrograph of the carrier of Example No. 3 of the invention at 50 magnification; FIGURE 7 is a photomicrograph of the carrier of Example No. 3 of the invention at 200 magnification; FIGURE 8 is a photomicrograph of the carrier of Example No. 4 of the invention at 50 magnification; FIGURE 9 is a photomicrograph of the carrier of Example No. 4 of the invention at 200 magnification; FIGURE 10 is a photomicrograph of the carrier of Example No. 5 of the invention at 50 magnification; and FIGURE 1 1 is a photomicrograph of the carrier of Example No. 5 of the invention at 200 magnification. DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention comprises a generally spherical shaped, magnetic carrier core powder which may be used for magnetic brush development in copy machines and laser printers. As taught in prior art patents such as those referenced above, magnetic carriers such as ferrites are used to transfer toner particles from a developer mix onto a photoreceptor. The particles are then transferred by the photoreceptor onto plain paper. The ferrite carrier powders are typically in the form of spherical beads or powder which may or may not be coated with resin. Also typically the ferrites are combined with various metal oxides which enhance the utility of the carrier powder. The present invention is a magnetic ferrite carrier powder which does not contain elements considered potentially hazardous such as nickel, copper, zinc and barium. Thus, the present invention comprises a generally non-stoichiometric lithium ferrite. Stoichiometric lithium ferrite composition may be represented by the following formulation: (Li 2 0). 26 (Fe 2 0 3 ). 26 Fe 2 0 3 Other ways of representing the stoichiometric formulation of the lithium ferrite composition include the following: 1 . LiFe 6 0 8 , or 2. Li 2 0 • 5 Fe 2 0 3 Lithium is monovalent and thus requires an equal molar amount of trivalent iron to obtain the desired spinel crystalline structure as a ferrite. Consequently, the formulas set forth above represent the stoichiometric composition of lithium ferrite. By contrast, the compositional range which is preferred or which is specified as comprising the present invention is represented by the following generally non- stoichiometric relationship: [(Li 2 0). 26 (Fe 2 0 3 ). 2B ] x {Fe 2 0,) 1Λ0< where .35 ≤ x ≤ .50 mole percent. Referring to Figure 1 , this composition range is represented by the cross-hatched portion of the ferrite/lithium ferrite phase diagram. The desired formulation of such a lithium ferrite powder material which constitutes a carrier has a spinel structure, is environmentally safe, and has the necessary characteristics to serve as an excellent carrier. Generally, the composition is prepared by the following sequential steps: 1 . Lithium carbonate or lithium oxide is mixed with iron oxide in the amounts prescribed by the compositional formula set forth above. The two compounds are intensely mixed by a wet or dry method. 2. The mixture of oxides is calcined to a temperature between 700° and 1 100°C as an optional step to prereact the mixture. 3. Calcined material or oxides from steps 1 and/or 2 are milled with water as a slurry in a milling unit such as an attritor or ball mill. To this slurry binders and deflocculants are added. Sintering aids may also be added to assist in densification and strength properties. Various other additives such as Si0 2 , Bi 2 0 3 , are typically added. This milling operation is ended when a desired particle size is achieved. 4. Slurry from the milling operation is spray dried to produce specified sized spheres referred to as beads. This operation is performed in a typical spray dryer using rotary or nozzle atomization. 5. Spray dried powder is screened to a specific size distribution in the green state. This operation is typically per ormed using a vibratory screening device. 6. Green screened product from the screening operation is sintered in a furnace or kiln in an atmosphere containing 21 % 0 2 capable of reaching temperatures of 1000°C to 1300°C. The degree of sintering depends upon the type of surface texture and apparent density desired. 7. The fired powder typically exhibits some degree of bead to bead fusion and is, accordingly, deagglomerated with a hammer type of mill. 8. Deagglomerated powder is screened to a specific size distribution. Air classification may be used for separation or screening finer particle distributions. 9. Magnetic separation may be performed as an option to ensure that no non¬ magnetic particles are contained in the powder product. 10. The final sintered powder may be coated with a resin coating to assist in the attainment of the desired reprographic properties. The present invention produces carriers with a variety of magnetic properties which may be used in different applications of magnetic brush development. The following is a table which sets forth the range of magnetic saturation as it correlates with the composition. Table 1 Mole Composition Magnetic Saturation (4000 Oe drive field) [(Li 2 0). 2B (Fe 2 O 3 ). 2B l. B0 (Fe 2 O 3 ). S0 or (Li 2 0). 1β7 (Fe 2 0 3 ). 833 61 .4 [(Li 2 0). 2B (Fe 2 0 3 ). 2B l. 4β (Fe 2 0 3 ). B4 or (Li 2 0). 14β (Fe 2 0 3 ). 8B1 60.6 [(Li 2 0). 26 (Fe 2 0 3 ). 2B ]. 42 (Fe 2 0 3 ). B8 or (Li 2 0). 133 (Fe 2 0 3 ). 8β7 44.4 [(Li 2 0). 2B (Fe 2 0 3 ). 2B l. 38 (Fe 2 0 3 ). β2 or (Li 2 0). 123 (Fe 2 0 3 ). 877 33.4 Set forth below are some specific examples of the lithium oxide ferrite carrier of the present invention, and a comparison thereof to typical commercially produced CuZn and NiZn ferrite materials. The carrier compositions are within the mole percentage range set forth in Figure 1 for the lithium oxide ferrite mixtures. The example carriers are thus of the nature and have a crystalline structure which is principally a spinel structure. Example No. 1 - Lithium ferrite according to the formulation (Li 2 0). 1B21 (Fe 2 0 3 ) 847β was prepared. Specifically, batch mixtures of 100 pounds including 7.67% by weight lithium carbonate and 92.33% by weight iron oxide were mixed. The batches were intensively dry mixed in an Eirich R-7 mixer/pelletizer. After pelletization, two (2) gallons of water was added to minimize dusting and promote pelletization of the raw oxides and carbonates. The pellets were oven dried and calcined in a batch electric kiln for four (4) hours at 1010°C. Calcined pellets were charged to a batch type steel ball grinding mill and milled six (6) hours, with the following additives: Table 2 After appropriate milling, twenty (20) lbs. of a 10% by weight polyvinyl alcohol (PVA) solution was added to the slurry to promote binding of the beads during spray drying. Airvol 205S brand of PVA was used. The slurry produced was nozzle atomized in a single fluid pressure nozzle type of dryer, using an 0.046 inch diameter orifice at 350psi to generate the appropriate bead size. Spray dried powder or beads resulting therefrom was classified using a 48" diameter Sweco brand vibratory separator with the acceptable mesh fraction being - 120 TBC Mesh, + 200 TBC Mesh (-149// + 88//). The resulting product was sintered at about 1 165°C for seven (7) hours in an air atmosphere in an electric fired batch kiln. Refractory boats were used to contain the powder during sintering. The resultant powder cake was deagglomerated in a hammer type mill, and product again screened in a 48" Sweco vibratory separator -145 TBC Mesh, + 250 Market Grade Mesh (-125 + 63//). The resultant carrier powder was then tested to determine its properties. Typical reprographic test properties are listed in Table 3. Figures 2 and 3 depict the physical appearance of the carrier at 50 and 200 magnification utilizing a scanning electron microscope (SEM). The separate core elements are noted to be generally uniform in size and spherical. Table 3 Comparison of Bare Core Properties Lithium Ferrite vs. Copper Zinc Ferrite and Nickel Zinc Ferrite σ> Example No. 2 - Lithium ferrite according to the formulation (Li 2 0). 14B (Fe 2 0 3 ) 0 8BB was produced using processing similar to that in Example No. 1 . The resulting test properties are listed in Table 3. Figures 4 and 5 depict the physical appearance of the carrier in SEM photomicrographs at 50 and 200 magnifications. These core elements are generally spherical and uniform in shape. Example No. 3 - Copper zinc ferrite of the formulation (CuO) 0 . 20 (ZnO) 0 .n (Fe 2 0 3 ) o ββ was produced using processing like that of Example No. 1 with the exception that the calcine temperature was 790°C and final sintering temperature was 1300°C. Measured test properties are listed in Table 3. Figures 6 and 7 are SEM photomicrographs of the described prior art carrier and is offered for purposes of comparison to the carrier of Example No. 1 and No. 2. The size, shape and appearance is very similar to to the lithium ferrite carriers. Example No. 4 - Copper zinc ferrite of the formulation (CuO) 020 (ZnO) 0 . 26 (Fe 2 0 3 ) 0 BB was prepared using similar processing as in Example No. 1 with the exception that the calcining temperature was 790°C and the final sintering temperature was 1 160°C. Measured test properties are also listed in Table 3. Figures 8 and 9 are SEM photomicrographs of another prior art formulation for a carrier and for purposes of comparison should be evaluated in relation to Figures 2, 3, 4 and 5. Again the comparison is one of high similarity. Example No. 5 - Nickel zinc ferrite of the formulation (NiO). 1Bβ3 (ZnO). 3220 (MnO). 02β3 (CuO). 01β0 (Fe 2 0 3 ). 47β3 was prepared using similar processing as set forth in Example No. 1 with the exception that the atomization occurred in a rotary atomization dryer and firing occurring at 1290°C. Figures 10 and 1 1 are SEM photomicrographs of this formulation and may be compared with the carriers of Figures 2, 3, 4 and 5. Measured test properties are listed in Table 3. Discussion of Examples A ferrite carrier core material composition preferably has several attributes to permit its use as a reprographic or electrographic carrier core material. For example, it should have the ability to adjust magnetic moment, Ms, similar to the carriers of Examples No. 3 and No. 4. This permits utilization in various copy machine designs. The described nonstoichiometric lithium ferrite carrier permits similar variations as set forth in Table 1 and for Examples No. 1 and No. 2. Bulk densities should be similar to the existing ferrite core materials. The lithium ferrite carriers of the invention have a bulk density very similar to that of existing ferrite core materials. Also, by changing sintering temperatures and soak time at temperature, bulk density may be varied higher or lower depending on the desired value. Flow rate determines the flow characteristics of a material in a copy machine magnetic brush developer station. Again, the lithium ferrite composition of the invention has very similar flow characteristics to that of pre-existing ferrite carriers. It is common for most carrier core materials to have either an acrylic, silicone, or fluoropolymer coating deposited on the carrier core surface to modify or enhance triboelectric or resistive properties for use with specific toners. For a new ferrite composition to comprise an acceptable substitute for existing coating technologies, it is important for surface texture, as measured by BEET surface area and visual observation by scanning electron microscopy, to show similar properties. Scanning electron microscopy analysis of Examples No. 1 through No. 5 demonstrates that the lithium ferrite carrier core of the invention is virtually indistinguishable from CuZn ferrite carrier core material and is similar to NiZn carrier core material. Comparison of BET surface area also shows very similar values. Also, BEET surface texture may be modified by adjustment of soak time, temperature, and processing conditions used to formulate the carrier core. Section 66699 of the State of California Administrative Code, Title 22, Division 4 lists offending elements that are (per soluble threshold limit concentration (STLC) and total threshold limit concentration (TTLC) limits) classified as a hazardous waste. Thus, depending on the composition, firing conditions and stoichiometry, it is possible, if not likely, for ferrite materials containing Ni, Cu, and/or Zn to fail either one or both of these test limits, and therefore such carriers will be classified as a hazardous waste and subject to appropriate and expensive disposal procedures. With the newly taught lithium ferrite material, offending elements are not present, and spent carrier materials may be classified as a benign waste. As such, they may be disposed or recycled very inexpensively. Thus, the applicants manufacture of lithium ferrite materials which have a range of non-stoichiometric compositions and a spinel structure are deemed to be materials which are environmentally safe. That is, such materials can be utilized safely to provide a magnetic brush for the carrying of toner particles, and when the material is expended or no longer useful, it can be easily disposed without constituting an environmental hazard. Various minor substitutions of constituents, additions of constituents and, of course, changes in the procedure for manufacture of the carrier are possible without departing from the spirit and scope of the invention. The invention is, therefore, to be limited only by the following claims and equivalents.

Claims: CLAIMS What is claimed is: 1 . An environmentally benign carrier for electrophotographic development comprising a non-stoichiometric lithium ferrite powder having a compositional range represented by the formula: t(Li 2 0). 2B (Fe 2 0 3 ). 2B ] x (Fe 2 O 3 ) 1 00 . x where .35 ≤ x ≤ .50 molar percent. 2. The, carrier of Claim 1 wherein the lithium ferrite powder in the carrier has a spinel crystalline structure. 3. The carrier of Claim 1 wherein the carrier is a generally spherically shaped magnetic core carrier. 4. The carrier of claim 1 where additions of Si0 2 or Bi 2 0 3 or both are included as densifying and strengthening elements. 5. The carrier of claim 1 which is resin coated. 6. The carrier of claim 4 which is resin coated.

Inventors: HUTCHESON WILLIAM R, SUKOVICH ALAN, HUTCHESON, WILLIAM, R., SUKOVICH, ALAN
Applicants: POWDERTECH CORPORTION
Application year: 1994
Main classification: G03G9
IPC codes: G03G9